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ADS1282-SP: unipolar operation

Knute Ray
Prodigy 230 points
Part Number: ADS1282-SP
Other Parts Discussed in Thread: ADS1282

Using the ADS1282 Delta Sigma ADC.  It is set up using a unipolar supply which the data sheet says is valid.  The supply is 5.0 volts (AVDD), AVSS is 0 volts.  The VREFP is 5 volts, the VREFN is 0 volts.
According to the data sheet, the input range is AVSS + 0.7  to AVDD -1.25 which is 0.7 volts to 3.75 V in unipolar mode (3.05 v range same as bipolar).
The data sheet has little information on unipolar operation.  Is the output code 80000000 for 0.7v and 7FFFFFFF for 3.75 volts?

  • 0
    TI__Mastermind 18900 points

    Hello Knute,

    I suggest you review this blog how to convert voltage to codes, it should help from the code side of things: https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/it-s-in-the-math-how-to-convert-adc-code-to-a-voltage-part-1 

    As for the inputs being AVSS+0.7V and AVDD-1.25V, these are relative to both specific inputs that are being applied. The ADC doesn't care about the absolute inputs directly, it cares about the difference between these two inputs. You can see that AINP - AINN = VIN in the table below. So, the absolute voltage on the AINP pin could be 0.7V (or GND really), and the same with AINN and our equations says:

    VIN = AINP - AINN

    VIN = (0.7V - 0.7V)

    VIN = 0V

    And you can find then you need to follow the math in the blog (or use the short hand in the table below) to transfer that VIN voltage to an actual code. In the case above VIN = 0V, is a code of 0V. If VIN is nonzero, then you'll definitely need to rely on the blog, reference voltage, and gain to get the code you expect. 

    By the way, the reason why the inputs should be between those two voltages is the result of op amp (or in this case, op amps in a PGA configuration) input limitations. You can find more info, here: https://training.ti.com/ti-precision-labs-op-amps-input-and-output-limitations-non-linear-behavior 

    Best,

    -Cole